Deletion analysis of molecular chimeras between the MMTV LTR and the v-ras transformation gene from Ha-MuSV, and between the LTR and the chloramphicol acetyl transferase (CAT) of Tn9, has localized the steroid target site in the MMTV LTR. The hormone regulatory sequences are located between 100 and 200 nucleotides upstream from the cap site. A negative as well as positive effect is suggested for the hormone regulatory element. Specific initiation of transcription at the MHTV cap has been demonstrated with MMTV LTR plasmids introduced into Xenopus Oocyte nuclei. The transformation phenotype of NIH 3T3 cells transformed with MMTV v-ras fusions is under the control of glucocorticoids, hormone dependent phenotype switching parallels the affinity curve for dexamethasone binding to the glucocorticoid receptor. This system is being exploited to monitor the interaction of the p21 transforming protein with potential cellular targets. A fourth gene (pLTR) is located at the 3 inch terminus of mouse mammary tumor virus (MMTV), within the long terminal redundancy (LTR). Sequence analysis has confirmed the maintenance of this reading frame in exogenous C3H-S MMTV, and in two endogenous proviruses of C3H, Mtv-1 (unit V) and unit II. A pLTR message uniquely encoding the information for this gene is expressed in preneoplastic mammary tissue (hyperplastic outgrowth, HOG) induced in Balb/c animals by the chemical carcinogen DMBA, and in lactating mammary tissue. The structure of this message suggests that it can be expressed as a second subgenomic MMTV transcript; in the Balb/c tissues, however, it is probably expressed from a partial endogenous provirus (unit I).